1. Field of the Invention
This invention relates to the spectral analysis of wood, and in particular to a method for predicting dry mechanical strength properties from the visible region of near infrared (NIR) spectra of green wood using a multivariate calibrations model, and is a continuation-in-part of U.S. patent application Ser. No. 09/738,912 filed Dec. 13, 2000, and entitled Method For Predicting Dry Mechanical Properties of Wet Wood and Standing Trees.
2. Description of the Prior Art
A method for the nondestructive analysis of the quality of a tree, unlike conventional methods, which measure the volume and form of a tree, would provide important information to assist woodland owners in making their thinning decisions, and in the valuation of a stand of timber. The method would also be useful in the analysis of trees or sawn logs, in the woods, for the field sorting of logs to be used as poles, or feedstocks in the manufacture of veneers, lumber or chips.
Visible and near infrared spectroscopy (VIS-NIR) in combination with multivariate data analysis is currently in use for the characterization of complex systems. These several statistical methods are also termed chemometric methods, forming the discipline of chemometrics, when applied generally to the field of chemistry, and in particular to the field of analytical chemistry. The technique of chemometrics is more fully explained in Brown, S. D., xe2x80x9cChemometicsxe2x80x9d, Anal. Chem. 62, 84R-101R (1990).
Near infrared spectroscopy and chemometrics have been described for use in the non-destructive analysis of the chemical and physical properties of paper.
For example, U.S. Pat. No. 5,638,284 describes a method for the measurement of the wet strength of paper by analyzing the visible, near-infrared and/or infrared spectrum of the paper/pulp in the process line using a wavelength range within 400 nm to 4,000 nm, and applying a chemometric evaluation of the spectrum, to calculate the wet strength of the paper. Other examples include U.S. Pat. No. 5,680,321 (determining physical properties selected from dry tensile strength, hydrophobicity, debonding energy, bursting strength, wettability and printability in paper), and U.S. Pat. No. 5,680,320 (quantifying the amounts of reacted and/or retained chemical additives in paper by analysis of the visible, near-infrared and/or infrared spectrum of the paper/pulp in a process line).
While the foregoing art discloses the use of chemometric evaluation in the analysis of paper products, the entire VIS-NIR spectral range between 400 nm and 4,000 nm is used for the evaluation. Also, the mechanical properties of wet-solid-wood samples are much more complex than those of paper due, in part, due to the presence of high concentrations of hemicellulose and lignin in wood relative to these components in paper. The structure and macromolecular morphology of the sample, such as roughness, color, and grain orientation also affect the spectral properties of solid wood. For a wet wood sample, the analysis of these properties is problematic because moisture in the samples, along with the high concentrations of lignin and hemicellulose tends to block or conceal the spectrometric derived information. Furthermore, many of these paper properties are a direct result of the presence of a small amount of an additive, or size or wet-strength resin, rather than a function of the inherent properties of paper fibers.
One example of the VIS-NIR characterization of wood is described in U.S. Pat. No. 5,965,888, in which, NIR spectrometric data are obtained from dried wood chips. The method for the determination of parameters of wood panels comprises analyzing the raw wood chips/panels at a moisture content  less than 10% by a spectrometric method to provide spectral data, and comparing the spectral data with reference spectral data from a reference chip/panel calibrated to known parameters of panels produced from the reference material, or of the reference panel by multivariate analysis. Again this method relies on the entire spectral range between 180 and 2,500 nm. This method is useful in predicting the quality of a dry wood panel based on an analysis of dried wood chips which are used as a feedstock in the manufacturing process.
VIS-NIR has also been used for determination of surface roughness and fiber angle of dry wood relative to the duration of the incident light, and for the evaluation of density and the strength of wood from a dry sample. See, e.g., Hoffmeyer, P., et al., Holz als Roh-und Werkstoff 53 (1995) 165-170 (density and strength from a dry sample).
In both U.S. Pat. No. 5,965,888 and Hoffmeyer, P., et al., Holz als Roh-und Werkstoff 53 (1995) 165-170, reference is explicitly made to the problems associated with measuring the NIR properties of wet wood, and seek to overcome them with use of a dry sample for analysis. All of these references use the full VIS-NIR spectral range, generally considered to be between 400 and 2,500 nm. Thus, they are using information from more than 2,000 individual wavelengths.
However, none of the foregoing references enables prediction of the dry mechanical strength of wet woody biomass, wood fibers, and various composite materials through the use of VIS-NIR measurements of wet wood coupled with a multivariate statistical calibration model (obtained from data derived from wet spectra together with known dry mechanical strength analytical results input into a computer, and either measuring a plot of modulus of elasticity (MOE) or modulus of rupture (MOR) of the known dry wood regressed against the MOE or MOR predicted by a multivariate model constructed with NIR spectra taken from the wet wood).
U.S. Pat. No. 5,945,676 discloses a method and apparatus for multi-spectral analysis in non-invasive VIS-NIR spectroscopy in which incident radiation containing a plurality of distinct, non-overlapping spectral regions of wavelengths is used to irradiate the sample. Diffusively reflected radiation emerging from the sample is detected, and a value indicative of the concentration of the analyte is obtained, preferably using an application of chemometric techniques.
A hand held device for infra red reflectance measurements of samples to identify the sample material and comprising a self-contained portable unit built into a hand held housing is disclosed in U.S. Pat. No. 6,031,233. The housing includes a window and optics on a bench adjacent the window, so that the optics are aligned with the sample when the device is placed directly against the sample. The optics include a broad-band IR light source shining onto an acousto-optic tunable filter (AOTF), which passes narrow-band IR light with a swept frequency; a lens focusing the IR through the window onto the sample; and a reflectance detector aligned with the window of the housing to pick up reflected light. A computer, which may be mounted in the housing, compares the detected reflectance spectrum with stored sample data spectra, and identifies the material or the components of the material and their proportions.
A need therefore exists to ascertain any advantages of VIS-NIR spectral sensitivity to simultaneously measure density, moisture content, slope in grain, microfibril angle, and other wood features, which when coupled with multivariate statistical analysis, will correlate the subtle spectral differences between wet wood samples to predict dry wood mechanical properties, such as ultimate bending strength or MOR and MOE.
There is a further need to utilize VIS-NIR from a reduced range of wavelengths to enable use of lightweight portable instrument means extensively available in the market to facilitate xe2x80x9con the spotxe2x80x9d or swift analysis of NIR spectra for predicting dry mechanical strength from standing trees or wet wood.
There is a need still further to provide a method for determining dry mechanical strength from standing trees or wet wood by the use of low-cost, light weight, spectrometers with vary rapid acquisition times through the use of spectrometers that are commercially available for both in-plant and in-field measurements, due to the fact that in-plant measurements require very rapid spectral acquisition times between 10 microseconds and 1 second, or more, for effective process monitoring, and in view of the fact that many spectrometers require moving parts and mechanical gratings which are too slow for most process control applications.
One object of the present invention is to provide a method to predict the mechanical strength of dry wood from green or wet wood VIS-NIR spectra from a calibration model using multivariate analysis.
Another object of the present invention is to provide a method which is useful in a manufacturing process, for quality control and process monitoring of a green feedstock or product, based on the mechanical properties of dry wood predicted from VIS-NIR spectra.
A further object of the present invention is to provide a rapid, accurate method for predicting the mechanical properties of standing trees, which is useful in assessing the value of a stand of timber, by quantitatively measuring the quality of the timber via VIS-NIR spectra.
A still further object of the present invention is to provide a method, which is useful in making timber thinning and harvesting decisions via VIS-NIR spectra.
An additional object of the present invention is to provide an apparatus for determining the dry mechanical strength of green wood.
The present invention overcomes the problems of the prior art methods by providing a method for predicting the dry mechanical strength for a green wood, comprising: illuminating a surface of the wood to be predicted, the wood having a green moisture content; analyzing the wood surface using VIS-NIR spectrometric method from a reduced range of wavelengths in the range of from about 400 to about 1,150 nm, the method generating a first spectral data; and using a multivariate analysis to predict the dry mechanical strength by comparing the first spectral data with a calibration model, the calibration model comprising a second spectrometric method of spectral data of a reduced range of wavelengths in the VIS-NIR spectra obtained from a reference wood having a green moisture content, the second spectral data correlated with a known mechanical strength analytical result obtained from a reference wood when dried and having a dry moisture content.
The present invention also provides analysis of green wood utilizing a reduced range of wavelengths of VIS-NIR to enable use of lightweight portable instrument means extensively available on the market to facilitate xe2x80x9con the spotxe2x80x9d analysis of VIS-NIR spectra for predicting dry mechanical strength from standing trees or wet wood.
In the context of the invention, the reduced range of wavelengths of VIS-NIR enables predicting mechanical and physical properties from standing trees or wet wood by taking VIS-NIR spectra from the radial face of a hole drilled into the wood from which VIS-NIR reflectance spectra are collected.
Also, in the context of the invention, both in-plant and in-field measurements that require very rapid spectral acquisition times between 10 microseconds and 1 second, or more, for effective process monitoring or field measurements is available through low-cost, lightweight spectrometers with very rapid acquisition times that are not available with spectrometers at VIS-NIR wavelengths above 1150 nm and associated with acquisition times of more than 3 seconds and therefore too slow for many process control applications.